This invention relates to devices for holding cutter bits used in road maintenance and, especially, in the mining of coal and other minerals. More particularly, it relates to those holding devices known as cutter bit support blocks, or bit holders, and mounting bases, or clevi, used to releasably hold said support blocks, and especially those arrangements where the support block is firmly fixed in the clevis by a locking mechanism attached to the clevis which abuts against the support block.
Recently, the coal mining industry has shown renewed interest in coal mining cutter bit holder systems which do not allow the bit support block to move in relation to the clevis in which it is located during mining operations. Movement of the support block within and against the clevis causes premature wearing of the support block and clevis, thereby reducing cutting efficiency and reducing the effective lifetime of the cutting drum or chain on which these bit holders are mounted.
In addition, when the clevis and support block are worn, the movement of the support block against the clevis produces a loud rattling noise in the mine during machine operation. Such noise produces, at the least, an uncomfortable working condition for the miners and, thereby, is a hinderance to achieving optimum productivity.
The above-mentioned bit holder systems firmly holding a bit support block in a clevis require the use of a locking mechanism as an integral part of a clevis. Examples of such arrangements are illustrated in U.S. Pat. Nos. 3,749,449; 3,397,013; 3,397,012; and 3,093,365. However, these arrangements share the common drawback that the locking mechanism, a threaded bolt, is held in a threaded perforation which is through the clevis itself. Having the locking bolt held in this manner is a drawback because, should the bolt break in the perforation, the threads on the perforation be stripped or corroded to the point where they can no longer tightly hold a bolt, the whole clevis is rendered useless and should be replaced.
Replacement of a clevis, if it is welded to a chain or drum, requires cutting it off with a torch or arc and welding on a new one in its place, if possible. If the clevis is itself a link in a continuous chain, the chain must be opened up to replace the clevis. Both manners of replacement are expensive due to the downtime, labor and replacement part cost required to affect a repair.
It should additionally be noted that in the prior art bit holding arrangements shown in U.S. Pat. Nos. 3,397,013; 3,397,012; and 3,093,365, the bolt was typically held horizontally or at a small rearward and downward incline in a threaded perforation in the clevis and abutted against a substantially vertical forward wall on the support block shank located in a uniform cross section vertical perforation in the clevis. This abutment between the bolt and the forward face of the support block forced a rear vertical wall of the support block into abutment with a rear vertical wall of the vertical perforation in the clevis. Where the forward wall of the support block was vertical, there was typically a shallow recess located in this wall into which the locking bolt entered.
These prior art bit holding arrangements did little or nothing to draw downward facing longitudinal shoulders of the support block into abutment with upward facing abutment shoulders on the clevis. Therefore, locking the support block into the clevis did not produce, or produced very little, upward preload on the support block shoulders to resist downward cutting forces produced during operation.
The bit holding arrangement shown in U.S. Pat. No. 3,749,449, FIG. 16, in addition to suffering from the disadvantage of having the locking mechanism as an integral part of the base member, relies upon abutment between the bottom surface of support block and the top sloping surface of the clevis to resist out of plane cutting forces, rather than abutment between shoulders on the support block and the clevis on both sides of the support block.